Electric heater and method of making the same



March 5, 1935. N. LIGHTFOOT 1,993,410

ELECTRIC HEATER AND METHOD OF MAKING THE SAME Filed June 2, 1932 2 Sheets-Sheet 1 51mm? SQMXP g 5? w 16 19 717 Jg March 5, 1935. LIGHTFOOT 1,993,410

ELECTRIC,HEATER AND METHOD OF MAKING THE SAME Filed June 2, 1932 2 Sheets-Sheet 2 Patented Mar. 5, 1935 UNITED STATES ELECTRIC HEATER AND METHOD OF MAKING THE SAME Edwin N. Lightfoot, Wauwatosa, Wis., assignor to Cutler-Hammer, Inc.,

poration of Delaware Milwaukee, Wis., a cor- Application June 2, 1932, Serial No. 614,886

15 Claims.

This invention relates to improvements in electric heaters and methods of making the same, and the invention relates more particularly to sheathed electric heaters of the strip type.

An object of the invention is to generally improve the structural and operative characteristics of electric heaters of the aforementioned type.

Another object is to facilitate production and assembly of the heater parts.

Another and more specific object is to provide an electric heater employing a resistor in the form of an elongated flattened helix, whereby a wide and efllcient resistor path is insured.

Another object is to provide novel means for temporarily centering and properly spacing the coils of the resistor.

Another object is to provide a method utilizing the means last mentioned to facilitate filling of the sheath with a granular, non-eementitious insulating material. v

Another object is to provide novel methods of reating the heater parts after assembly thereof to provide a finished article and to aiford a proper electrical test thereof.

Other objects and advantages of the invention will hereinafter-appear.

The accompanying drawings illustrate an embodiment of the invention which will now be described, it being understood that the embodiment illustrated is susceptible of modification in the details thereof without departing from the scope of the appended claims.

In the drawings, Figure 1 is a top plan view of a finished electric heater of the strip type, as

constructed in accordance with my invention,- a part of the sheath being broken away to show the form and arrangement of the helical resistor.

Fig. 2 is a side elevational view of the heater shown in Fig. 1, with a part thereof shown in cross section.

Fig. 3 is a fragmentary perspective and cross sectional view of a finished heater, showing the flattened helix form of the resistor and the manner in which the latter is spaced fromthe walls of the two-part sheath or casing by the highly compacted insulating material.

Fig. 4 is a fragmentary plan view of one of the sheath parts, showing the insulated arrangement .of the wiring terminal and resistor supporting elements and the manner of attaching the resistor thereto.

Fig. 5 is a similar view, on a reduced scale, showing the fibrous supporting threads and the manner of tying the coils of the helix thereto (or operation with the bolt shank being omitted for clarity of illustration.

Fig. 8 is a side elevational view of a partially assembled heater positioned vertically to facilitate the filling operation, and

Fig. 9 is a fragmentary view at right angles to Fig. 8, and showing in dotted lines the manner in which the cover plate is bent to overlapping relation to effect closure of the heater casing.

Referring to the drawings, the numeral designates one of thepunched and stamped sheet metal parts of the sheath or casing-said part being formed to provide a recess 15- of substantially rectangular form both in transverse and longitudinal cross section, and the same having relatively narrow longitudinal flanges 15 and relatively wide end flanges 15. Recess 15 is provided in its bottom wall adjacent to'each end thereof with relatively large openings 15 which are preferably of circular contour (Figs. 6 and 7). Located within said ends of the recess 15 are plates 16 of insulating material, such as mica; each of said plates having a bent end portion l6 extending to a point substantially flush with the open face of the recess, and parallel side portions 16 which initially extend beyond said open face of the recess (see Fig. 7). Plates 16 are provided with openings 16 just suiiiciently large to accommodate the threaded shank 17 of a bolt having a flat head 1'7 with parallel straight side edges 17 ,--a punched and stamped sheet metal member 18 being'interposed between each flat head 17 and the insulating plate 16. Member 18 is provided with parallel arms 18* and an end arm 18', said arms being adapted for clamping engagement with side portions 16 and end portion 16 of plate 16 when member 18 is pressed into the recess 15. Member 18 will thus be held against rotary movement relatively to the walls of said recess.

Member 18 is also provided with an opening 18 to provide clearance for shank 17 of the bolt, and lugs 18 are sheared and bent upwardly (as viewed in Fig. 4) to provide shoulders 18 to re- 1 celve therebetween the edges 1'1 0! bolt head 17' to prevent rotary movement of said head with respect to member 18. 11 desired, the lugs 18 may ,be bent in the reverse direction into gripping engagement with the edges 17 to restrain the bolt against accidental separation from member 18 during assembly of the heater parts. Rigidly attached to the fiat head 1'7 of the bolt, as by spot-welding at the point indicated by numeral 19 is a punched and stamped sheet metal member 19, said member being of substantially T-shape, and having the relatively wide portion 19 thereof offset from the fiat head 17 of the bolt, said portion 19 having rearwardly extending lugs 19 to which the coil supporting threads are attached, 'as hereinafter described. Member 19 is provided with a lug or extension 19 which is of reflexed or hook-shape, as best illustrated in Fig. '2. One end of the resistor wire 20 is inserted between the arms of extension 19, and said arms are then pressed together, as by means of a pair of pliers, to provide a good mechanical and electrical connection between the same and said Wire end.

A mica insulating washer 21 is strung upon the shank 17 of the bolt, and said washer fits within the opening in the bottom wall of recess l5 at one end of the latter. A relatively larger mica washer 22 (Fig. 6) is then strung upon shank 1'7, then a metal washer 23, and a nut 24 which is adapted to clamp all of the parts of the terminal to the sheath part. A washer 25 having an annular flange extending downwardly over nut 24 is provided with a cut away portion to accommodate the end of a circuit wire (not shown), said washer being clamped in position by tightening of nut 26. The terminals at each end of the casing part are of identical form and are attached in a like manner. Before tightening the nut 24 a strip 27 of mica is inserted beneath the edge of plate 16,-said strip acting to insure against any possibility of contact between the coils of the helix and the bottom wall of recess 15 in the area between the respective terminal element and the portion of the sheath or casing which is subjected to the die-pressing operation hereinafter described. Strip 2'7 may, if desired, be formed integrally with plate 16, or said strip may be entirely omitted.

As aforestated, the resistor wire is wound into the form of an elongated flattened helix as shown at 20 in the various figures of the drawings. This wire is preferably composed of a known alloy of nickel and chromium which is adaptable for use as a heater resistor; and the helix while still upon the winding mandrel is preferably heat-treated or annealed to fix the form thereof and to prevent any tendency toward twisting or deformation thereof after removal thereof from the mandrel. The helix is placed under a slight degree of tension as the.

ends of the wire are attached to the'terminal elements in the manner aforedescribed,it being noted that the relatively wide portions 19 of members 19 fit into the opposite ends of the helix to assist in centering the latter and to hold or two-longitudinally extending portions of the same thread, are thus arranged in spaced relation to each other, and at longitudinally spaced points the coils of the helix are attached or tied to said threads by small knotted loops of thread, as indicated at 29. It will thus be apparent that when the helix is positioned vertically the several intermediate portions thereof will be sup ported by the threads 28 to prevent sagging or unequal spacing of the coils of the former.

When the terminal members and the helix are thus positioned within and attached to sheath part 15, a cover plate 30 is attached to the latter, the cover plate 30 having one of its ends 30 bent over one of the end flanges 15. of sheath part 15, and having its longitudinal edges 30" bent over a major portion of the length of flanges 15 to provide a seam connection between the sheath parts. As shown in Fig. 8, however, a portion of the seam connection is left uncompleted and the unattached end of cover plate 30 is bent or bowed outwardly to provide an opening at that end of the sheath. The sheath is then positioned vertically upon a suitable support, represented diagrammatically at 31.

Thereupon the granular, non-cementitious insulating material is introduced by gravity into the sheath through the open upper end thereof, as by means of a funnel 32. The support 31 will preferably be vibrated vertically to a slight degree to insure complete filling of the sheath.

While various granular, non-cementitious insulating materials have been found practical, I prefer to employ commercially pure granular quartz, or quartz sand,not only because of the comparative cheapness of this material, but also because of the good heat conducting and dielectric properties thereof when treated in the manner herein contemplated by me.

'When the sheath has been filled, the remainder of the edge portions 30b of cover plate 30 are bent into overlapping relation to flanges 15 and the end 30 is bent over flange 15, as shown in dotted lines in Fig. 9, to completely close the sheath. Thereafter the heater is placed in a pressure die and subjected to upwards of one hundred tons pressure between the points indicated at 33 and 34 in Fig. 2. This portion of the sheath is thus reduced in thickness and the granular insulating filling is crushed and highly compacted, whereby the resistor helix is positively fixed in spaced relation to the inner walls of the sheath, whereas the compacted insulation affords a good path for conduction of heat from the resistor. I prefer to employ a hydraulic press for the compacting operation, and it will be understood that the pressure of one hundred tons is to be employed for heaters having a length of approximately twelve inches, whereas for heaters having a length of twenty-four inches approximately one hundred and eighty tons die pressure will be required.

After the heater has been treated in the manner aforedescribed, I prefer to subject the same to a heat treatment to burn or consume the silk thread or threads whereby all harmful traces of the latter are eliminated from the flnished article. Said heat treatment may consist in passing through the resistor a current having a value considerably in excess of that for which the heater is rated. Such treatment at the same time will serve as a final manufacturing test of the heater whereby a given maximum current carrying capacity thereof is insured when the of the heater to a suitable support.

While I have described in detail hereinthe method of and means for. producing an electric heater of the strip type, it is to be understood that certain features of the invention have utility as applied to electric heaters of other types.- -Also it will be apparent that by slightly changing the size of the sheath the same may be made to accommodate more than one resistor. Similarly it will be apparent that by slight modification of the heater parts the wiring terminal elements may both be positioned at one end of the sheath. Various other changeswill be at once apparent to those skilled in the art.

WhatI claim as new and desire to secure by Letters Patent is:

1. In an electric heater, in combination, a metal jacket, a resistor to be enclosed by said jacket, said resistor being in the form of a relatively long helix, a pair of combined supporting and wiring terminal elements attached to said jacket in insulated relationship to the latter and to which the opposite ends of said helix are electrically and mechanically connected, and 'a plurality of combustible fibrous threads attached to said supporting elements and extendingunder tension therebetween, said threads being attached to certain of the intermediate coils of said helix, whereby said intermediate coils are supported by said threads and in substantially equally spaced relationship to each other upon vertical positioning of said jacket for the purpose set forth.

2. In an electric heater, in combination, a resistor comprising a wire wound into the form of a coreless helix having parallel flat sides, a twopart sheet metal sheath providing an elongated,

closed, box-like chamber, terminal elements secured to and extending laterally from one of said sheath parts and having portions thereof located within said chamber at opposite ends thereof, the opposite ends of said helix being electrically and mechanically connected to said portions of the terminal elements, means for temporarily main taining the respective convolutions of said helix in substantially equally spaced relation upon endwise positioning of said sheath, said means com prising a plurality of combustible threads attached to and extending between said portions of the terminal elements, said threads being attached to said helix at a multiplicity of spaced points throughout the length of the latter, and a dry, granular, non-cementitious insulating material adapted to fill the space within said chamber not occupied by said helix.

3. In an electric heater, in combination, a twopart sheet metal sheath providing a closed chamber of substantially rectangular form both in transverse and longitudinal cross section, terminal members located at opposite ends of said,

chamber and having wiring terminal portions extending laterally through one of the metal parts but insulated therefrom, a helically wound wire coil located within said chamber, said coil having flat substantially parallel sides and rounded edge portions providing a coreless helix, the opposite ends of said coil being electrically and mechanically connected to said terminal'members, and common means attached to said terminal members and to said wire coil at a multiplicity of spaced points throughout the length of the latter to insure substantially equal spacing of the respective convolutions thereof upon endwise positioning of the sheath for the purpose set forth.

4, In an electric heater, in combination, a twopart sheet metal sheath providing a closed chamber of substantially rectangular form both in .transverse and longitudinal cross section, terminal members located at opposite ends of said chamber and having wiring terminal portions extending laterally through one of the metal parts but insulated therefrom, a helically wound wire coil located within said chamber, said coil having a pair of relatively wide, fiat substantially parallel sides and a pair of relatively narrower edge portions providing a coreless helix, the opposite ends of said coil being electrically and mechanically connected to said terminal members, and common means attached to said terminal members and to said wire coil at a multiplicity of spaced points throughout the-length of the latter to insure substantially equal spacing of the respective convolutions thereof and centering of said convolutions with respect to the chamber walls upon vertical positioning of the sheath to facilitate introduction of a granular refractory insulating filling.

5. In an electric heater, in combination, a twopart sheet metal sheath providing a closed chamber of substantially rectangular form both in transverse and longitudinal cross section, terminal members located at opposite ends of said chamber and having wiring terminal portions extending laterally through one of the metal parts but insulated therefrom, a helically wound wire coil located within said chamber, said coil having a pair of relatively wide, fiat substantially parallel sides and a pair of relatively narrower connecting edge portions, the opposite ends of said coil being electrically and mechanically connected to said terminal members, and means attached to said terminal members and to said wire cell at spaced points throughout the length of the latter to insure substantially equal spacing of the respective convolutions thereof and centering of said convolutions with respect to the chamber walls upon vertical positioning of the sheath to facilitate introduction of a granular refractory insulating filling, said last mentioned means consisting of combustible material, for the purpose set forth.

6. In an electric heater, in combination, a twopart sheet metal sheath. providing a closed chamber of substantially rectangular form both in transverse and longitudinal cross section, terminal members located at opposite ends of said chamber and having wiring terminal portions extending laterally through one of the metal parts but insulated therefrom, a helically wound wire coil located within said chamber, said coil having a pair of relatively wide, flat substantially parallel sides and a pair of relatively narrower connecting edge portions, the opposite ends of said coil being electrically and mechanically connected to said terminal members, and means attached to said terminal members and to said wire coil at spaced points throughout the length of the latter to insure substantially equal spacing of the respective convolutions thereof and centering of said convolutions with respect to the chamber walls upon vertical positioning of the sheath to facilitate introduction of a granular refractory insulating filling, said last mentioned means con- 5 sisting of a combustible material; whereby said material is adapted to be burned and substantially eliminated upon passage of an electric current of suitable value through said wire coil.

7. A resistor supporting and wiring terminal assembly for electric heaters of the sheath type,

said head and adapted to directly receive and to which comprises a bolt having a flat head to be positioned within the sheath of the heater, a stamped sheet metal member welded to said flat head, said member having a portion offset from center one end of a resistor of helical form, and said member having another portion of hook form .between the arms of which one end of the resistor wire is adapted to be received, said arms being bendable toward each other to clampingly engage said wire, end.

8. A resistor supporting and wiring terminal assembly for electric heaters of the sheath type, which comprises a bolt having a flat head to be positioned within the sheath of the heater, a

stamped sheet metal member welded to said flat head, said member having a portion offset from said head and adapted to directly receive and to center one end of a resistor of helical form, said member having another portion of hook form between the arms of which one end of the resistor wire is adapted to be received, said arms being bendable toward each other to clampingly engage said wire endfmeans for supporting said bolt in insulated relation to the sheath of a heater, said means comprising a nut having engagement with the threaded shank of the bolt exteriorly of the sheath, and a second nut having threaded engagement with said shank and adapted for clamping 0 engagement with a suitable conductor.

9. In an electric heater of the strip type, in combination, a metal sheath part formed to provide a recess, the bottom wall of said recess having an opening formed therein, a flat-headed bolt having its shank penetrating said opening and exposed on the outer surface of said sheath part, a metal member interposed between said fiat head and the bottom wall of said recess, said member having parallel lugs cooperating with but insulated from the side walls of said recess to prevent relative rotation of the former, said member also having lugs engaging the flat head of said bolt to prevent relative rotation of the latter, means associated with the shank of 5 said bolt in insulated relation to said sheath part a to provide for attachment of a circuit wire, and means fixed to said fiat head to provide for mechanical support and electrical connection of a resistor wire, said means including a T-shaped lug offset from the plane'of said head to accommodate the end coils of a helix and a hook-shaped lug arranged at substantially right angles to said head, the arms of said last mentioned lug being bendable into clamping engagement with the end of said resistor wire.

said helix, positioning said parts horizontally to equalize the spacing of the respective coils of the resistor, attaching said helix to said tie member at a multiplicity of spaced points throughout the length of the former, whereby said helix will be supported within said sheath in a manner to provide substantially equal spacing of the re-.

spective coils thereof from each other and from the walls of said sheath upon vertical positioning of the latter, introducing a filling of granular refractory insulating material into said sheath while the latter is so positioned, and thereafter compressing said sheath flatwise under heavy die pressure to compact said insulating filling where- 'by said helix is permanently supported in spaced relation to said sheath walls.

11. The steps in the method of making an electrio-heater having a sinuous resistor, a metal sheath and a filling of granular refractory insulating material, which consist in electrically and mechanically connecting the resistor under tension to longitudinally spaced supporting elements within said sheath, arranging said sheath in a horizontal position to equalize the strain upon and spacing of the intermediate bends or convolutions of said resistor, extending at least one combustible fibrous thread between said supporting elements, and attaching the intermediate portion of said resistor to said thread at a plurality of longitudinally spaced points, whereby said intermediate portion will be held against sagging upon vertical positioning of the sheath to facilitate introduction of the granular refractory insulating material.

12. The method of making an electric heater of the strip type, which comprises winding a resistor wire into the formof a relatively long fiat helix, attaching terminal elements at spaced points'within a metal sheath member of substantially channel-shape, electrically and mechanically connecting the ends of said helix to said terminal elements, attaching one or more tie members to said terminal elements to extend therebetween, attaching said helix to said tie members at a plurality of spaced points throughout the length of the former, attaching a flat sheath member to said first-mentioned sheath member throughout the major portion of the length thereof, said flat member being bent outwardly to provide an opening at one end of the sheath, vertically positioning said sheath with the closed end thereof at the bottom, introducing granular refractory insulating material into said sheath to fill the same while so positioned, then completely closing said sheath, and thereafter compressing the opposed fiat sides of said sheath under heavy pressure to crush and compact said insulating material, whereby said helix is fixed in properly insulated relation to the walls of said sheath.

13. The method of making an electric heater of the strip type, which consists in forming two sheet metal parts to provide a casing having a relatively long chamber of substantially rectangular form in transverse cross section, locating within said chamber a resistor wire wound in the form of a coreless helix having parallel flat sides, connecting the opposite ends of said helix under tension to supports arranged in spaced relation to the walls of said chamber and connecting certain intermediate coils of said helix to tie members extending between said supports whereby said helix will be spaced and centered with respect to said walls and whereby said intermediate coils are restrained against sagging upon endwise positioning of said casing, filling said chamber with loose granular insulating material while said helix is so centered, and then subjecting the opposite flat sides of said casing to upwards or one hundred tons die pressure accordingto the length of the latter, whereby said insulating material is crushed and compacted to improve the heat conductivity thereof and to maintain said helix in a ilxed relation to said chamber walls.

1d. The steps in the method oi making an electric heater oi the strip type having a resistor in the form of an elongated flattened helix, a twopart metal sheath and a filling of granular refractory insulating material, which consist in electrically and mechanically connecting the opposite ends of the helix to terminal members located at longitudinally spaced points on one of the sheath parts, arranging said part in a horizontal position to equalize the strain upon the individual convolutions of said helix, extending at least two fibrous, combustible threads under tension in laterally spaced relation be-= tween said terminal members and attaching the same to the latter, and then tying certain of the convolutions of said helix to the respective threads at a number of spaced points on the latter, whereby upon vertical positioning of said sheath part the intermediate convolutions of said helix will be held against sagging.

15. The method of making an electric heater of the strip type, which comprises stamping a metal sheath member to provide therein an open recess of elongated rectangular form, attaching terminal elements to the bottom wall or said recess at opposite ends thereof and in insulated relation heater to the influence of heat thereto, winding a wire into the form of an elongated helix, having parallel flat sides and rounded edge portions, electrically and mechanically connecting the ends of said helix to said terminal elements, stretching a, plurality of combustible fibrous threads between and attaching the same to said terminal elements, attaching certain of the convolutions of said helix to said threads at a plurality of longitudinally spaced points while said helix is in a horizontal position, whereby said threads may be utilized to support the individual convolutions in substantially equally spaced. re lationship when said helix is positioned vertisally, applying a fiat metal cover to said'sheath member and bending the edges of said cover-into overlapping relation to said sheath member to unite said parts, the sheath so formed being provided with an opening at one end thereof, yer tically positioning said sheath with the opening at the upper end thereoi', introducing by gravity through said opening a sufllcient quantity of noncementitious refractory insulating material to fill said sheath, thereafter closing said opening, then subjecting the heater so formed to heavy die pressure to reduce the thickness thereof, whereby the insulating material is compacted around and within the coils of the helix to fix the latter in position and to improve the heat conductivity and the dielectric characteristics said insulating material, and thereafter subjecting said eilect combustion and substantial elimination of said threads and to substantially eliminate or neutralize any torsional strains within said wire helix.

EDWIN N. LIGH'I'FOOT. 

